Reference herein to an "ink eradicator" will be understood t mean a substance which is capable of decolorizing an ink rather than a substance which removes the ink from the substrate paper to which the ink is applied. This is, of course, the common meaning of an ink eradicator and thus the term "ink eradicator" and the expression "ink decolorizer" may be used interchangeably herein.
The inks currently in use include those based upon triaryl methane dyestuffs. It is known (see, for example, German Patent Document DE-OS 2216951) to provide an ink decolorant for such dyestuffs which comprises an aqueous solution of a decolorizing substance in the form of a reducing sulfur compound and, if desired, a binder and other additives which may be conventional in ink eradicators.
The sulfite contained in this aqueous solution and serving as the reducing sulfur compound is capable of decolorizing written symbols, imprints or marks for the purposes of correction.
The ink eradicator solution generally comprises a relatively high proportion of the reducing sulfite. In this case it has been found to be necessary to substitute an ink containing another dyestuff when the corrected region must be overwritten since the remaining sulfite on the corrected region of the substrate tends to decolor a newly applied ink when the latter also contains the original dyestuff, i.e. a triaryl methane dyestuff.
As a consequence, it has been proposed to significantly reduce the sulfite proportion in the ink eradication solution. Low sulfite quantities in the ink eradication solution, however, give rise to a poor shelf life of the product, i.e. a failure of the ink eradicator to function after long term storage, since the sulfite is converted into ineffective sulfate by the action of air or oxygen.
In Europatent publication 0032652, this problem is attacked by incorporating a basic nitrogen compound in small concentrations as the decolorizing substance and by adjusting the pH value so that the nitrogen compound is present as the free base.
The basic nitrogen compound, which functions as a Lewis base, like the sulfite, eliminates the problem of the poor shelf life of a sulfite containing ink eradicator. Since the basic nitrogen compounds must be present as free bases, the pH value of the ink eradicator solution must be in a more or less strongly alkaline range. The high alkalinity of such solutions can result, upon application of the ink eradicator solution to the paper region to be corrected, in damage to the paper structure, so that upon overwriting the paper fibers may be pulled out and the desired sharp correction and high resolution cannot be achieved.
Reference may also be made to German Patent Document DE-AS 1111760 which discloses the presence of glycerine in a solution used for ink eradication.
By and large, all of these prior art systems have not been fully satisfactory for one or more reasons. The product may not have satisfactory shelf life; the product may not be capable of The product may not be capable of satisfactorily decolorizing the ink symbols to be corrected; the product may damage the paper, etc.